The acquisition of a matrix-assisted laser desorption-time of flight (MALDI-TOF) Mass spectrometer is essential to the advancement of biomedical research at the University of California, San Diego. Recent advances in spectrometer development have made this technology the method of choice for sequences tryptic fragments of novel proteins to facilitate cloning and further study. Picomolar quantities of novel proteins can be rapidly sequenced, and the gene from which the protein was expressed can be rapidly cloned. Similar tryptic digestion experiments an also be used to identify post-translational modifications, namely phosphorylation and glycosylation, that may be stable or may occur transiently during phases of the cell cycle, or during signal transduction. Again identification, of these modifications on minute quantities of proteins can be rapidly obtained only by MALDI- TOF mass spectrometry. Recent experiments by researchers at UC San Diego have demonstrated another novel use of MALDI-TOF mass spectrometry, measurement of amide hydrogen/deuterium exchange rates that may be too rapid to measure by other techniques. By this novel technology, MALDI- TOF enables the rapid identification of protein-protein interfaces in protein-protein interactions, narrowing the search for the functionally important region of novel proteins discovered by interaction technologies such as yeast two-hybrid. This technology can also be used to study protein conformational changes and protein folding. These experiments have all been carried out via collaborative arrangements because there is currently no MALDI-TOF mass spectrometer at UC San Diego. It is now critical for the advancement of these experimental methods that a research-grade MALDI-TOF be obtained here at UC San Diego, one with the highest resolution attainable, so that it will be possible to distinguish the presence or absence or deuteriums on large proteins and protein fragments. The proposed instrument would be used 75% for research experiments and 25% of the instrument time would be devoted to analysis of samples submitted by researchers other than the research users. The instrument shall be located in the Macromolecular Biophysics Laboratory central to the departments of Chemistry and Biochemistry, Biology, and the School of Medicine, in which the researchers reside. Access to sample analysis shall be provided to everyone at UC San Diego. Four of the research users are members of the UCSD Cancer Center, and technical support for sample analysis has been committed by the Cancer Center.